1. Field of the Invention
The present invention relates to an improved method for operating a transmitting/receiving circuit as well as to a device utilizing the method.
2. Description of the Prior Art
For range metering of an object from a transmitter, it is known to measure the running time of a transmitter signal. In this connection, the use of a phase-locking circuit (PLL) for detecting the signal echo has advantages as for instance a low control resolution with high immunity from noise at the same time which makes useful the application of such a PLL circuit within the receiving portion. If such a PLL circuit is used as a trigger circuit for detecting the running time, inaccuracies of the measurement can result at a triggering on the leading edge, since lock-in of the phase-locking depends on the random phase of the received signal with respect to the phase of the free-running oscillator of the PLL circuit. The error appearing at triggering on the leading edge basically does not exist at the lock-out of the PLL circuit. As soon as the frequency of the received signal is not within the lock-in range of the PLL circuit or as soon as the amplitude of the received signal is below the detectable amplitude, the PLL circuit locks-out.
In the case of ultrasonic transmitters, piezo-ceramic sound transducers are commonly used for producing ultrasonic waves. Those transducers have basically a relatively small bandwidth with respect to their inherent frequency, i.e., in the transmitting mode the radiated spund pressure declines quickly outside the inherent frequency, and at the reception of a sound pressure signal the efficiency of the transducer quickly decreases outside the inherent frequency. From this behavior, different problems result in the event where an ultrasonic signal is switched-on for a certain time in the transmitting mode, and where the end of the received signal is detected in the receiving mode.
On the one hand, such transducers after their external excitation are still oscillating over a certain period, i.e., the output amplitude shows an exponential behavior of an attenuated oscillation. Since the echo amplitude may vary by a factor of 1000 or more depending on the range of the object to be measured, the phase-locking circuit locks-out later with a high amplitude rather than with a low amplitude or with a near object distance rather than a far object distance. This influence of the absolute echo amplitude on the lock-out point forms an essential error source. By a standardization of the signal by means of an automatic gain control as has been earlier provided a certain leveling may be achieved; however, a further error remains. The oscillations of a sound transducer excited exactly with its inherent frequency decline slower than an oscillation which is achieved by an excitation other than the inherent resonance of the sound transducer. With the dependency of temperature of the inherent frequency of the known piezo-ceramic sound transducers and their narrow-band behavior, the exact inherent frequency at the excitation of the transducer may be easily missed which results in a variation of the steepness of the trailing edge of the radiated signal, and therefore, results in a further system error.
By a forceable attenuation of the amplitude of the sound transducer after transmitting, for instance by means of driving with a counter-phased signal, as shown in U.S. Pat. No. 4,654,833, it is possible to achieve a quick decline of the radiated amplitudes in the range of high amplitudes. However, a controllable zero amplitude is not practically attainable within short time. Even if this would be possible, the decline of the received amplitude would be leveled down at the reception of an echo from a near object since in the near range the high acoustical received signal would excite the sound transducer to oscillations of relative high amplitude after the acoustical excitation has finished.